1. Field of the Invention
The present invention relates to a switching power supply and an electronic apparatus including a switching power supply, and more particularly, to a switching power supply that operates with low loss even under a low load condition, and an electronic apparatus including a switching power supply.
2. Description of the Related Art
In switching power supplies such as an RCC switching power supply, the switching frequency changes depending upon whether a load is low or high. When the load is high, the on-period and the off-period are both increased, and thus the switching frequency decreases. On the other hand, when the load is low, the on-period and the off-period are both decreased, and thus, the switching frequency increases. Various types of loss occur in the switching power supply. One type of loss is a switching loss which occurs each time a switching device switches. Because the switching loss occurs each time the switching device switches, if the frequency increases with decreasing load, a large switching loss occurs. Thus, the switching loss is a major component of the loss of the switching power supply when the load is within a low range. When the load is very low, intermittent oscillation may occurs, which causes an increase in output ripple or noise that is generated.
Japanese Unexamined Patent Application Publication No. 2001-16849 discloses a technique for reducing the increase in the switching frequency during the operation under a low load condition, by reducing the switching rate during the operation under the low load condition. In this technique, a minimum on-period of a switching device is set, and, when the output voltage increases as the load decreases, a signal which controls the turning-on of the switching device is blocked so as to prevent the switching device from turning on, thereby preventing the on-period of the switching device from decreasing to less than the minimum on-period, whereby the switching rate during the operation under the low load condition is reduced to achieve a reduction in switching loss.
However, in the switching power supply disclosed in Japanese Unexamined Patent Application Publication No. 2001-16849, the signal which turns the switching device on is blocked to prevent the turning-on of the switching device only when the output voltage is greater than a set value, and thus, the off-period of the switching device and the switching frequency are not constant even under a static load condition. Therefore, the problems associated with the increase in the output ripple and noise are not adequately solved.